Cryptochromes are a family of flavoproteins that function as blue light photoreceptors in plants and animals. Drosophila Cryptochrome functions as a deep-brain, non-ocular photoreceptor for photic entrainment of circadian rhythms. In vivo, Cryptochrome mediates light-dependent degradation of the circadian clock gene product Timeless;by a separate mechanism, it also triggers its own degradation following light exposure. We have established an in vitro assay for studying light-dependent Cryptochrome degradation in cell culture, by fusing the bioluminescence enzyme luciferase to full-length Cryptochrome. Greater than 80% of luciferase activity is lost within one hour of light exposure. The conferral of light-triggered degradation to a functional protein fused to Cryptochrome is a potentially very powerful way to study the function of specific proteins in cells. We will determine the minimal fragment of Cryptochrome necessary to mediate light-dependent degradation, and determine the range of proteins which can be made light-labile by fusion with Cryptochrome. We will apply this method to the study of proteins in whole flies. We will identify proteins required for light- mediated protein degradation, and attempt to use these components to port this system to mammalian cell culture. Results from these studies will enhance our understanding of the mechanisms of light-mediated protein degradation, and will additionally provide a valuable new method for studying specific proteins'function in vitro and in vivo. Project Narrative: Cryptochromes are blue light photoreceptors found in plants and animals. Drosophila Cryptochrome undergoes light-dependent degradation in vivo and in vitro. Fusion proteins containing Cryptochrome also undergo light-dependent degradation. We propose experiments to employ this phenomenon as a general tool for studying protein function.